The present invention relates to a method and an apparatus for disinfecting drainage, and more particularly, to a method and an apparatus for disinfecting sewage diluted with rainwater.
In sewage works, sewage is subjected to treatment in a sand basin for removing sand, etc., solid-liquid separation for removing suspended solids (SS), activated sludge treatment, and disinfection in this order, and then discharged to public waters such as rivers, lakes, ports, and coastal waters.
Disinfection generally involves the use of a chlorine gas or a chlorine-based disinfectant, because sewage, night soil, industrial drainage, etc. may contain pathogens which cause infectious diseases. Generally, the chlorine-based disinfectant is added to such drainage to be treated, thereby decreasing the number of coliform organisms (coliform organism count) to 3,000 CFU (colony forming unit)/ml or less. CFU refers to a colony forming unit. Alternatively, ultraviolet irradiation or ozonization may be performed without the addition of the chlorine-based disinfectant. Since such a technique requires vast equipment, however, its applications are limited.
A combined sewer is a system for collecting household waste water, industrial drainage, and rainwater into the same pipe, and sending the combined water to sewage works, where the aforementioned treatments are carried out. When there is much rainfall, rainwater-incorporating sewage in excess of the amount that can be treated at sewage works is likely to flow into the sewage works. Thus, discharge takes place from (wet-weather) sewage removal facilities, such as a storm outflow and pump station, to public waters. Recently, techniques for preventing overflow of bulky waste, floating matter, etc. by providing sewage removal facilities with screens have begun to be studied to protect the scenery of public waters such as rivers. However, no studies have been performed on techniques for disinfecting coliform organisms included at a count of several tens of thousands to several hundreds of thousands in water discharged from sewage removal facilities.
A separated sewer is a system for collecting both of household waste water and industrial drainage, and rainwater into different pipes, and sending the household waste water and industrial drainage to sewage works, while discharging the rainwater as an overflow. The separated sewer overflow should essentially comprise only rainwater. Actually, however, when much rain falls, a large amount of rainwater flows in the sewer. On this occasion, pollutants present on ground surfaces, such as roads, and sludge deposited in the sewer are flowed together. Thus, the separated sewer overflow also contains Escherichia coli ascribed to the pollutants existent on ground surfaces and the sludge. In each case, the coliform organism count in the overflow may exceed the discharge control value (3,000 CFU/ml or less). In this case, disinfection is desired.
Chlorine-based disinfectants have many advantages, such that the equipment used is simple, and their applicability to any state of dirt is high, compared with ultraviolet irradiation and ozone sterilization.
However, when the techniques applied to ordinary sewage treatment are diverted to disinfection of combined sewer overflow, the following problems arise: In sewage in rainy weather, ammonia or amine is coexistent. Thus, a chemical reaction typified by the chemical equation (1) indicated below takes place. As a result, active chlorine is converted to chloramine, decreasing a microbicidal effect to one-tenth or lower. Hence, in the presence of ammonia or amine, the amount of the chlorine-based disinfectant added needs to be increased, even if the pathogen count is unchanged.
NH4++HClOxe2x86x92NH2Cl+H2O+H+xe2x80x83xe2x80x83(1)
The disinfection time for the use of the chlorine-based disinfectant is required to be 15 minutes or more (see xe2x80x9cSewer Facilitiesxe2x80x94Plan and Design Guidelines and Descriptionxe2x80x9d). Thus, there is need for a mixing tank in which sewage in rainy weather and the chlorine-based disinfectant are mixed and caused to dwell for 15 minutes or more. However, the (wet-weather) sewage removal facilities have no ample space where such a mixing tank can be installed.
Thus, a disinfectant taking a short disinfection time, and a method for mixing it are required of disinfection of combined sewer overflow.
J. E. Alleman, J. E. Etzel, D. E. Gendron, J. C. Conley, W. F. McCoy, and A. J. Hein, Bromine-Based Disinfection Performance, a paper by researchers of Purdue University and Great Lakes Chemical Company, reports on laboratory-scale experiments in which bromine chloride (BrCl), bromine (Br2), and bromochlorodimethylhydantoin (BCDMH) were each added to dummy drainage containing bacteria such as coliform bacilli. As the dummy drainage, water at pH 7.2 containing a low concentration of ammonia (2 mg/L), or water at pH 8.2 containing a high concentration of ammonia (20 mg/L) was used. As the bacteria, Escherichia coli, Pseudomonas, and Streptococcus faecalis were used. However, the paper does not describe the dummy drainage as containing organic matter.
Japanese Unexamined Patent Publication No. 4-156994 describes a method of pouring a germicide into cooling water. As regards the germicide, the formation of hypobromite ions by a redox reaction between ozone and bromine ions is described. However, the cooling water contains no ammonia.
Japanese Unexamined Patent Publication No. 11-47755 describes a slime control agent containing a hydantoin compound as an active ingredient, and a slime control method using such an agent. The slime control agent is used in storage water for use at a pulp plant or a paper making factory.
According to an aspect of the present invention, there is provided a method for disinfecting drainage, comprising the steps of mixing a disinfectant, which can form HOX where X is a bromine atom or an iodine atom, and which contains a bromine atom or an iodine atom, with water to obtain disinfecting water; and adding the disinfecting water to the drainage containing organic matter and ammonia or ammonium ions to disinfect the drainage.
In the invention, the total organic carbon in the drainage is preferably 5 mg/liter or more. The ammonium ion concentration in the drainage is preferably 1 mg/liter or more.
The drainage preferably includes rainwater. The drainage also preferably includes sewage diluted with rainwater.
The disinfectant preferably contains a 4- to 10-membered heterocyclic ring which may be condensed with other ring and which contains 1 to 4 hetero-atoms comprising nitrogen atoms or sulfur atoms. The heterocyclic ring preferably includes a group of the formula xe2x80x94N(X)xe2x80x94C(xe2x95x90O)xe2x80x94, where X includes a bromine atom or an iodine atom, in a ring skeleton. Furthermore, it is preferred that the disinfectant be a solid, and the step of obtaining disinfecting water should include the step of dissolving the disinfectant in the drainage.
The concentration of the disinfectant in the disinfecting water is preferably 100 mg/liter as Cl to 10 g/liter as Cl calculated as an active chlorine concentration.
The concentration of the disinfectant added in the drainage is preferably 0.5 mg/liter as Cl to 25 mg/liter as Cl calculated as an active chlorine concentration.
The step of adding the disinfecting water preferably includes the step of introducing the disinfecting water below the water surface of the drainage. It is also preferred that the step of discharging the disinfected drainage to public waters be further included.
According to another aspect of the invention, there is provided an apparatus for disinfecting drainage, comprising a device for producing disinfecting water from a disinfectant and the drainage; a sand basin for removing sand in the drainage; and a first channel for introducing the disinfecting water into the sand basin, wherein the drainage is disinfected while the drainage is dwelling in the sand basin.
In the invention, the device for producing disinfecting water preferably has a disinfectant storing device, a device for adding the disinfectant to the drainage, and a device for mixing the disinfectant and the drainage. Preferably, the sand basin has two or more sand settling portions, and the first channel has a distribution tank for introducing the disinfecting water to each of the sand settling portions.
The first channel is preferably connected to an adding device for introducing the disinfecting water below the water surface of the drainage.
It is preferred that a reservoir for storage, or a discharge waterway be further included so that the disinfected drainage can be discharged to public waters.
The reservoir or the discharge waterway is preferably provided with a measuring instrument for inspecting the water quality of the disinfected drainage.
It is preferred that a second channel for introducing part of the drainage in the sand basin into the device for producing disinfecting water be further included.
Preferably, the above disinfectant can form HOX, where X is a bromine atom or an iodine atom, and contains a bromine atom or an iodine atom.
The disinfectant also preferably contains a 4- to 10-membered heterocyclic ring which may be condensed with other ring and which contains 1 to 4 hetero-atoms comprising nitrogen atoms or sulfur atoms.